The lower the ambient temperature is, the higher the demand for the heat capacity of an air conditioner is. However, current heat pump with single-stage compression can only be started normally for operation at −20° C. (outdoor ambient temperature) or above. Further, the heat capacity is seriously attenuated, thus the heating effect cannot be guaranteed, and reliability of the unit also undergoes a severe test.
Compared with the heat pump with single-stage compression, a two-stage compression system with enhanced vapor injection has a greater heat capacity and a higher energy efficiency at low temperature. Further, the two-stage compression system can reduce the pressure ratio of the single-stage compressor and reduce exhaust temperature, and can meanwhile increase suction efficiency and compression efficiency to thereby increase the heat capacity and the heating efficiency.
In the related two-stage compressor, as the volume of the high pressure cylinder and the volume of the low pressure cylinder are fixed, their volume ratio is a fixed value, and thus the ratio of the displacement of the high pressure stage to that of the low pressure stage of the compressor is fixed. However, any compressor with a fixed ratio of displacement has only one highest value of energy efficiency at a specific working condition, and with a variation in air-conditioning load, energy efficiency varies greatly, resulting in unideal economic performance. On the other hand, under the nominal refrigeration conditions, the optimal compression ratio of the high pressure stage to the low pressure stage of the two-stage compression system is around 0.9, whereas under the low-temperature heating condition, the compression ratio of the high pressure stage to the low pressure stage is optimally around 0.6. Therefore, the two-stage compression system with a fixed compression ratio cannot simultaneously meet the energy efficiency requirements for refrigeration and heating.
The related two-stage compression system can only achieve a variation in displacement ratio of the high pressure stage to the low pressure stage by series connection of two inverter compressors, in which the system is complicated and high in costs, and oil return is low in reliability.